The process for the production of acrolein most commonly used is based on the gas-phase catalytic oxidation reaction of propylene by atmospheric oxygen, such as described, for example, in the document Techniques de l'ingénieur, traité Génie des procédés [Techniques for the Engineer, Process Engineering Treatise], J 6 100 1-4. The acrolein thus obtained can either be incorporated directly in a two-stage process for the manufacture of acrylic acid from propylene in the gas phase or be used as synthetic intermediate. Acrolein is in particular a key intermediate in the synthesis of methionine, a synthetic amino acid used as animal food supplement which has emerged as a substitute for fishmeal. Acrolein also has numerous other applications in preparing derivatives which can be synthesized on the actual site of production of the acrolein, thus limiting the storage and transportation of this toxic chemical product.
In a certain number of cases, it may be advantageous to be able to increase the acrolein production capacities of existing units.
The production of acrolein is highly dependent on the starting material, propylene. Propylene, obtained by steam cracking or catalytic cracking of petroleum fractions, has the disadvantage of contributing to the increase in the greenhouse effect, as a result of its fossil origin. Furthermore, propylene resources may become limited.
It thus appears particularly advantageous to be able to increase the productive output of acrolein while reducing dependence on a fossil resource.
It has been known for a long time that glycerol can contribute to the production of acrolein. Glycerol results from the methanolysis of vegetable oils, at the same time as the methyl esters, which are themselves employed in particular as fuels in diesel oil and heating oil. This is a natural product which enjoys a “green” aura, is available in a large amount and can be stored and transported without difficulty.
Numerous studies are devoted to giving an economic value to glycerol according to its degree of purity, and the dehydration of glycerol to give acrolein is one of the routes envisaged.
The reaction involved in producing acrolein from glycerol is:CH2OH—CHOH—CH2OHCH2═CH—CHO-2H2O
Generally, the hydration reaction is promoted at low temperatures and the dehydration reaction is promoted at high temperatures. In order to obtain acrolein, it is thus necessary to employ a satisfactory temperature and/or a partial vacuum in order to displace the reaction. The reaction can be carried out in the liquid phase or in the gas phase. This type of reaction is known to be catalyzed by acids. Various processes for the synthesis of acrolein from glycerol are described in the prior art; mention may in particular be made of the documents FR 695931, U.S. Pat. No. 2,558,520, WO 99/05085 and U.S. Pat. No. 5,387,720.
It has now been found that the reaction for the dehydration of glycerol to give acrolein can be carried out in the presence of a propylene-comprising gas. It is thus advantageous to introduce glycerol into the process for the gas-phase catalytic oxidation of propylene, which makes it possible to use a renewable starting material while increasing the production of acrolein. Such a process becomes particularly advantageous for the synthesis of methionine, which can then be said to be “obtained from biomass”. This is because the methionine, when it is used in the feeding of animals, is rapidly metabolized and the carbon dioxide gas, which is reencountered in the atmosphere, contributes to increasing the greenhouse effect. If the acrolein is obtained partially from a renewable starting material, such as the glycerol originating from vegetable oil, the CO2 emissions no longer entirely participate in the balance of the process as they offset the carbon dioxide gas used by the biomass for its growth; there is thus a limitation on the increase in the greenhouse effect. Such a process then corresponds to the criteria associated with the new concept of “green chemistry” in the more general context of sustainable development.